Clinical use of any antiviral drug can lead to the development of drug-resistant viruses (Pillay et al., 1998; De Clercq, 2004). Two neuraminidase (NA) inhibitors, oseltamivir and zanamivir, have proven effective against influenza and are used extensively to combat this infection, especially in Japan (Ward et al., 2005; Roche, 2005). There is documentation of the emergence of oseltamivir-resistant type A viruses, including H5N1 subtypes (Ward et al., 2005; Kiso et al., 2004; Le et al., 2005; de Jong et al., 2005), but similar information on influenza B viruses with reduced sensitivity to NA inhibitors is limited. Although influenza B viruses usually cause smaller epidemics than type A viruses, they are nonetheless associated with annual outbreaks of illness and excess mortality rates worldwide (Treanor et al., 2005).
Of the two type B viruses with reduced sensitivity that have been reported, one carried an Arg152Lys mutation (amino acid numbering system adapted for an N2 NA, see Colman et al., 1993; N2 numbering is used herein) in its NA and was isolated from an immunocompromised child treated with zanamivir (Gubareva et al., 1998). The other had an Asp198Asn NA mutation and was isolated from an immunocompromised child treated with oseltamivir (Gubareva, 2004). The known NA substitutions identified in drug-resistant viruses from humans tend to be type- or subtype-specific: Glu119Val, Arg292Lys and Asn294Ser in the NA of the N2 subtype, His274Tyr in the N1 subtype (including not only H1N1 viruses but also H5N1 viruses) (Le et al., 2005; de Jong et al., 2005), and Arg152Lys and Asp198Asn in the NA of type B virus (Gubareva et al., 1998; Gubareva, 2004). All of these substitutions have been identified at catalytic or framework residues in the sialidase active site of the NA protein (Colman et al., 1993), which are relatively conserved in all type A and type B NA molecules and are the targets of NA inhibitors.
The results of cell culture experiments in which multiple passages were required for the generation of NA inhibitor-resistant viruses (McKimm-Breschkin, 2000) suggested that resistance to these agents arises infrequently. It is thus reasonable that a low frequency of oseltamivir resistance, 5.5% for children aged 1-12 years infected with type A viruses and none in children infected with type B virus, was observed in a clinical trial (Whitley et al., 2001). However, more recent studies demonstrated a higher-than-expected rate of drug-resistant influenza A virus generation in oseltamivir-treated children: 18% of children with H3N2 virus infection and 16% of those with H1N1 virus infection (Ward et al., 2005) harbored resistant variants with NA mutations after drug treatment.
Very little is known about the frequency of generation and transmissibility of influenza B viruses with reduced sensitivity to neuraminidase (NA) inhibitors. Further, transmission of resistant variants, whether type A or B virus, has yet to be shown.